Obtains the comparison key for the character c, such that all characters that are equivalent to this character in the imbued locale, ignoring the case differences, if any, produce the same key.

When the regex library needs to match two characters c1 and c2 and the flag std::regex_constants::icase is true, it executes regex_traits<>::translate_nocase(c1)== regex_traits<>::translate_nocase(c2).